This invention relates generally to laser fiber amplifiers and, more particularly, fiber amplifiers employing stimulated Raman scattering (SRS) for optical mode discrimination. The principle of SRS is widely known and used in connection with laser amplifiers. U.S. Pat. No. 6,363,087 B1 issued in the name of Robert R. Rice and entitled “Multimode Raman Fiber Amplifier and Method,” discusses the prior art of Raman fiber amplifiers. This patent, which will be referred to for convenience as “the '087 Rice patent,” discloses a device and method that have objectives similar to those of the present invention.
For a variety of both military and commercial applications, there is a need for laser amplifiers that can efficiently convert electrical power to high brightness laser output. A commonly proposed solution is to use electrical power to energize a high brightness laser diode pump array, the output of which is used to excite an appropriate rare earth dopant ion, such as neodymium (Nd) or ytterbium (Yb), in a crystal or fiber host to provide optical gain for a desired wavelength. The excited rare earth dopant establishes the wavelength for laser operation and converts some of the pump photon energy into heat, which reduces efficiency and produces deleterious temperature gradients in the host medium. In the device of the '087 Rice patent, the diode pump radiation produces optical gain by a nonlinear process, stimulated Raman scattering, which occurs at a wavelength shifted down in energy by a small amount (approximately 300-500 cm−1) from the photon energy of the pump. This small energy loss is significantly smaller that the energy lost in the conventional pumping process, leading to higher potential efficiency, less heat deposited in the host medium, and flexibility in operating wavelength, which is determined solely by the pumping source.
Raman oscillators are available as commercial products, but all are single-mode devices, which require pumping by single-mode pump sources. The '087 Rice patent disclosed a fiber of dual-clad design, including a uniform single-mode core, a surrounding Raman pump core and a low index outer cladding. The pump signal, which is presumed to be multimode, is launched or focused into the Raman pump core of the fiber amplifier and, at least in theory, a single-mode output is obtained. In practice, mode discrimination in the device is highly dependent on the ability to launch the pump signal “cleanly” in a lowest order mode. Some mode discrimination is also provided by attenuation of higher order modes due to scatter that occurs at the interfaces between the central core and the Raman core and between the Raman core and the outer cladding layer.
Accordingly, there is still a need for improvement in the field of Raman fiber amplifiers. Specifically, there is a need for a Raman fiber amplifier that reliably produces a single-mode output from the multimode pump laser input. The present invention is directed to this end.